Considering the following two histories: $H_1=\{W_2(x),W_1(x),R_3(x),R_1(x),W_2(y),R_3(y),R_3(z),R_2(x)\},$ and

$H_2=\{R_2(z),R_3(x),W_2(y),W_1(x),R_1(x),W_2(x),R_3(z),R_3(y)\},$ where $x,y,z$ are separately located on different sites, $R_i$ means a read

operation from transaction $i.$ Similarly, $W_i$ means a write operation from transaction $i.$ Which of the above transactions is$($are$)$ serializable? Please explain.